Electric potential measuring apparatus, and image forming apparatus

ABSTRACT

An electric potential measuring apparatus including a substrate disposed facing a measurement object, a detecting electrode provided insulated from the substrate, and a capacity modulating unit for modulating a coupling capacity between the detecting electrode and the measurement object. In the electric potential measuring apparatus, a region with a dielectric constant less than a dielectric constant of the substrate is formed in the substrate to achieve an increase in resistivity of a portion of the substrate between detecting electrodes, or a decrease in a stray capacity between the detecting electrode and the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric potential measuringapparatus capable of measuring an electric potential of an object to bemeasured (a measurement object) based on the amount of electrical chargeinduced in a detecting electrode, and an image forming apparatusincluding the electric potential measuring apparatus, applicable as acopying apparatus, a printer, and the like.

2. Description of the Related Background Art

Conventionally, there exists an image forming apparatus which includes aphotosensitive drum and forms an image in an electrophotographic manner.In such an image forming apparatus, the photosensitive drum needs to beuniformly charged in any atmosphere so that a stable image can beobtained at all times. To achieve the above purpose, the chargedelectric potential of the photosensitive drum is measured by an electricpotential measuring apparatus, and a feedback control is executed usingthe measured result to maintain a uniform electric potential of thephotosensitive drum.

As an electric potential measuring apparatus usable for performing thefeedback control, there has been developed an electric potentialmeasuring apparatus capable of measuring an electric potential of ameasurement object in a non-contacting manner in which any substantialmember of the electric potential measuring apparatus is not in contactwith a surface of the measurement object.

Description will be given for a potential measuring principle of theelectric potential measuring apparatus. Upon generation of an electricfield between a surface of a measurement object and a detectingelectrode in an electric potential measuring apparatus, charges with anelectric amount Q proportional to an electric potential V of the surfaceof the measurement object are induced in the detecting electrode. Therelationship between Q and V is written byQ=CV  (1)where C is the electrostatic capacity between the detecting electrodeand the surface of the measurement object. Pursuant to equation (1), theelectric potential of the surface of the measurement object can beobtained by measuring the electric amount Q of charges induced in thedetecting electrode.

It is, however, difficult to accurately and directly measure theelectric amount Q induced in the detecting electrode within ameasurement time required by a copying apparatus, a printer, or thelike. Accordingly, a practical method is used. In the practical method,the magnitude of the electrostatic capacity C between the detectingelectrode and the surface of the measurement object is periodicallychanged, and the electric potential of the surface of the measurementobject is obtained by measuring an AC current signal generated in thedetecting electrode by the periodical change.

A principle of obtaining the electric potential of the surface of themeasurement object by the above-discussed method will be described. Whenthe electrostatic capacity C is a function of time t, the AC currentsignal i generated in the detecting electrode can be represented byi=dQ/dt=d(CV)/dt  (2)since the AC current signal i is a value of a time derivative of theelectric amount Q induced in the detecting electrode, and the equation(1) holds.

Where a changing speed of the electric potential V of the surface of themeasurement object is sufficiently slow relative to a changing speed ofthe electrostatic capacity C, the equation (2) can be replaced byi=V·dC/dt  (3)since the electric potential V can be assumed to be constant during ashort time dt.

From the equation (3), it can be understood that the magnitude of the ACcurrent signal i generated in the detecting electrode is a linearfunction of the electric potential V of the surface of the measurementobject. Therefore, the electric potential of a measurement object can beacquired by measuring the amplitude of an AC current signal.

As a method of periodically changing an electrostatic capacity C betweena detecting electrode and a surface of a measurement object, there aretypically three methods: (1) a method of periodically changing aneffective area of the detecting electrode exposed to the surface of themeasurement object, (2) a method of periodically changing a relativedielectric constant between the detecting electrode and the surface ofthe measurement object, and (3) a method of periodically changing adistance between the detecting electrode and the surface of themeasurement object. This is because the electrostatic capacity C isapproximated by the following equation (4),C=A·S/x  (4)where A is the proportional constant having connection with a dielectricconstant of a material between the detecting electrode and the surfaceof the measurement object, and the like, S is the area of the detectingelectrode, and x is the distance between the detecting electrode and thesurface of the measurement object.

In a situation of the above-discussed conventional technology, thephotosensitive drum is down-sized and a structure around thephotosensitive drum becomes dense. Accordingly, reduction in size andthickness of an electric potential measuring apparatus is also required.In conventional electric potential measuring apparatuses, a space in theelectric potential measuring apparatus is occupied almost by assemblagemembers of a vibrating cantilever, a driving mechanism for vibrating thecantilever, and the like. Therefore, small-sizing of those assemblagemembers is indispensable for purposes of reducing the size of theelectric potential measuring apparatus.

However, when the driving mechanism and the like are reduced in size,the amount of change in the exposed area S of the detecting electrode,or in the distance x between the detecting electrode and the surface ofthe measurement object inevitably decreases. Here, from the aboveequations (3) and (4), the magnitude of a current taken out as an outputsignal from the above-discussed electric potential measuring apparatusis written asi=V·d(A·S/x)/dt  (5)Therefore, when sizes of the driving mechanism and the like are to bedecreased, a value of the time derivative in parentheses in the equation(5) becomes small. As a result, the current signal i of the outputsignal is likely to be affected by noise from outside, and a measurementprecision disadvantageously lowers.

Considering those discussed above, a small-sized electric potentialmeasuring apparatus produced using MEMS (micro-electro-mechanicalsystem) techniques has been recently proposed (see U.S. Pat. No.6,177,800). The MEMS techniques are techniques for fabricating a micromechanical mechanism or electric device by utilizing semiconductormicro-processing techniques for large-scale integration and the like. Byusing the MEMS techniques, it is possible to mass-produce micromechanical mechanisms integrated with electric devices, and the like,and largely reduce the size and cost of an electric potential measuringapparatus.

An advantage of a method of fabricating an electric potential measuringapparatus by the MEMS techniques is as follows. Reduction in the sizeand cost of an electric potential measuring apparatus can be achieved byfabricating, on a substrate, a small-sized driving mechanism, adetecting electrode, and a signal processing unit for processing asignal generated in the detecting electrode, which are componentelements of the electric potential measuring apparatus. Semiconductorwith an intentionally-enhanced carrier concentration is often used asthe substrate such that electronic circuits for signal-processing andthe like can be constructed on the substrate.

It is, however, known that resistivity of the semiconductor with a highcarrier concentration is low. Therefore, when a material with a lowresistivity is used as the substrate, it is likely that non-negligiblestray capacity appears between the substrate, and the detectingelectrode and electric wire formed on the substrate through aninsulating thin film. Hence, almost all of AC signals generated in thedetecting electrode or electric wire flow into another detectingelectrode or electric wire through the stray capacity. This phenomenonoccurs in all detecting electrodes and electric wires similarly.Therefore, an AC signal in a detecting electrode mixes with a drivingsignal for driving the driving mechanism, for example. Thus, measurementof an accurate electric potential value is likely to be prevented.

Difficulty of conduction of an AC signal with a frequency f between adetecting electrode or electric wire, and another detecting electrode orelectric wire, i.e., an absolute value |Z| of impedance Z therebetween,is given by|Z|={(ar)²+[1/(2pf)·(1/C _(h)+1/C′ _(h))]²}^(1/2)  (6)where C_(h) is the stray capacity between a detecting electrode and thesubstrate, C′_(h) is the stray capacity between another detectingelectrode and the substrate, r is the resistivity of the substrate, anda is a proportional constant. It can be understood from the equation (6)that the absolute value |Z| of the impedance Z among detectingelectrodes and electric wires should be increased in order to reduce oreliminate mixture of AC signals among detecting electrodes and electricwires. In other words, it is necessary to increase the resistivity r ofa portion of the substrate between detecting electrodes, and/or todecrease the stray capacity C_(h) between the detecting electrode andthe substrate.

SUMMARY OF THE INVENTION

The present invention is directed to an electric potential measuringapparatus and an image forming apparatus including the electricpotential measuring apparatus.

According to one aspect of the present invention, there is provided anelectric potential measuring apparatus which includes a substrate to bedisposed facing a measurement object, a detecting electrode provided onthe surface of substrate and facing the measurement object, and acapacity modulating unit configured to modulate a coupling capacitybetween the detecting electrode and the measurement object. In theelectric potential measuring apparatus, the substrate is composed of amaterial with a first dielectric constant. The substrate includes aregion formed therein having an average value of a dielectric constant,which is less than the first dielectric constant.

According to another aspect of the present invention, there is providedan electric potential measuring apparatus which includes a substrate, aninsulator, a plurality of detecting electrodes disposed on the substratewith the insulator therebetween, a capacity modulating unit configuredto modulate a coupling capacity between the detecting electrodes and themeasurement object, and a detecting unit configured to detect anelectric potential of the measurement object based on a signal detectedby the detecting electrodes. In the electric potential measuringapparatus, an insulator portion is formed in at least a portion of thesubstrate such that an electric impedance between the detectingelectrodes is increased.

As the capacity modulating unit, it is possible to use a construction inwhich a vibrating plate supported by at least a torsion bar in aswingingly-rotatable manner is vibrated relative to the measurementobject such that a distance between the detecting electrode provided onthe vibrating plate and the measurement object can be modulated, aconstruction in which a shutter with an opening is moved in a spacebetween the measurement object and the detecting electrode provided on afixed substrate such that an area of the detecting electrode exposed tothe measurement object can be modulated, or the like.

According to another aspect of the present invention, there is providedan image forming apparatus which includes the above-described electricpotential measuring apparatus of the present invention, and an imageforming unit. In the image forming apparatus, a portion thereof with thedetecting electrode is arranged facing an object surface to be measuredin the image forming unit, and the image forming unit controls imageformation based on a signal detected by the electric potential measuringapparatus. The image forming unit can have a function of copying,printing, facsimile, or the like. For example, the image forming unitincludes a photosensitive drum rotatable about a predetermined axis, andis so constructed as to measure a charged potential of a surface of thephotosensitive drum by using the electric potential measuring apparatus.

In an electric potential measuring apparatus of the present inventionwhich includes a substrate to be disposed facing a measurement object, adetecting electrode provided under a condition insulated from thesubstrate, and a capacity modulating unit configured to modulate acoupling capacity between the detecting electrode and the measurementobject, the stray capacity appearing among detecting electrodes andelectric wire can be reduced or eliminated, or the resistivity r of thesubstrate can be increased.

The features of the present invention will be more readily understood inconnection with the following detailed description of the embodimentsand examples of the invention in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an electric potentialmeasuring apparatus of a first embodiment according to the presentinvention.

FIG. 2 is a cross-sectional view of the first embodiment.

FIG. 3 is a cross-sectional view illustrating an electric potentialmeasuring apparatus of a second embodiment according to the presentinvention.

FIG. 4A is a perspective view illustrating an electric potentialmeasuring apparatus of a third embodiment according to the presentinvention.

FIG. 4B is a cross-sectional view of the third embodiment, taken alongline 4B–4B′ of FIG. 4A.

FIG. 4C is another cross-sectional view of the third embodiment.

FIGS. 5A to 5C are cross-sectional views illustrating variousarrangements of portions with a reduced dielectric constant in asubstrate of the third embodiment according to the present invention,respectively.

FIG. 6A is a plan view illustrating an electric potential measuringapparatus of a fourth embodiment according to the present invention.

FIGS. 6B to 6E are cross-sectional views, taken along line 6B–6B′ ofFIG. 6A, illustrating various arrangements of insulator portions in asubstrate of the fourth embodiment, respectively.

FIG. 7A is a plan view illustrating an electric potential measuringapparatus of a fifth embodiment according to the present invention.

FIGS. 7B to 7D are cross-sectional views, taken along line 7B–7B′ ofFIG. 7A, illustrating various arrangements of insulator portions in asubstrate of the fifth embodiment, respectively.

FIG. 8A is a plan view illustrating an electric potential measuringapparatus of a modification of the fifth embodiment according to thepresent invention.

FIGS. 8B and 8C are cross-sectional views, taken along line 8B–8B′ ofFIG. 8A, illustrating various arrangements of insulator portions in asubstrate of the modification of the fifth embodiment, respectively.

FIG. 9A is a plan view illustrating an electric potential measuringapparatus of a sixth embodiment according to the present invention.

FIGS. 9B to 9D are cross-sectional views, taken along line 9B–9B′ ofFIG. 9A, respectively.

FIG. 10 is a view schematically illustrating a seventh embodiment of animage forming apparatus according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of an electric potential measuring apparatus and an imageforming apparatus of the present invention will hereinafter be describedwith reference to the drawings.

A first embodiment directed to an electric potential measuring apparatuswill be described with reference to FIGS. 1 and 2. As illustrated inFIGS. 1 and 2, a detecting electrode 103 for detecting a signal isdisposed on a substrate 100 through an insulator layer 102. Ameasurement object 105 is placed above the detecting electrode 103, anda capacity modulating unit 104 for modulating the amount of chargesinduced in the detecting electrode 103 is provided in a space betweenthe detecting electrode 103 and the measurement object 105.

The detecting electrode 103 for detecting a signal is connected to asignal detecting circuit (CKT) via an electric wire 106. A signalamplifier 107 for amplifying the signal supplied from the detectingelectrode 103 is provided in the electric wire 106. A portion or region101 with a reduced dielectric constant is formed in a portion of thesubstrate 100. Due to the presence of the region 101 with a reduceddielectric constant, it is possible to increase the resistivity r of aportion of the substrate between detecting electrodes, and/or todecrease the stray capacity C_(h) between the detecting electrode andthe substrate. Thus, a compact electric potential measuring apparatuswith an improved performance can be achieved.

In this embodiment, the region 101 with a reduced dielectric constant isa region whose dielectric constant is less than the dielectric constantof the other portion of the substrate 100. For example, where thesubstrate 100 is a semiconductor substrate, the region 101 with areduced dielectric constant can be composed of an insulator. Morespecifically, a silicon substrate with a dielectric constant of about11.9 can be used as the semiconductor substrate, and a polyimide with adielectric constant of about 3.2 can be used as the insulator.

A material of the region 101 is not limited to an insulator. It can becomposed of whatever material or structure whose dielectric constant isless than that of the semiconductor substrate. The region 101 can beformed of a semiconductor material different from the semiconductor ofthe substrate 100, an oxide or nitride of a semiconductor material, anorganic compound, a porous material, or the like.

The substrate 100 can be formed of a semiconductor, such as Si, GaAs,and GaN, but it can also be composed of an electrically-conductivematerial, depending on the structure of the apparatus. The insulatorlayer 102 can be composed of an oxide, such as SiO₂ and Al₂O₃, but itcan also be an oxide film almost spontaneously formed on a surface ofthe semiconductor (such as Si) substrate. Further, the insulator layer102 can be formed of a porous material of a nitride of Si, SiO₂, Al₂O₃,or the like, depending on the structure of the apparatus.

The detecting electrode 103 can be composed of whateverelectrically-conductive material, such as Cu, Pt, Ag, Au, and Al. Thecapacity modulating unit can be a chopper for changing the area of asurface facing the measurement object 105, as illustrated in FIG. 1. Thea capacity modulating unit can also be a unit for changing the distancebetween the detecting electrode 103 and the measurement object 105.

The measurement object 105 can be composed of whatever material orcomponent that has an electric potential. Specifically, it can be aphotosensitive drum, or a paper on which an electric potential isgenerated by friction with a roller or the like. The electric wire 106can be formed of anything that can supply a signal from the detectingelectrode 103 to the signal detecting circuit (CKT). When the signal issmall, the signal can be amplified by the signal amplifier 107.

Description will now be given for the principle of achieving a decreasein the stray capacity, or an increase in the resistivity r of thesubstrate owing to the above-discussed construction.

A stray capacity C_(h) between a detecting electrode and a substrate isapproximated by the following equation (7),C _(h) =e ₀ ·k _(h) ·S _(h) /d _(h)  (7)where k_(h) is the relative dielectric constant of a material betweenthe detecting electrode and the substrate, S_(h) is the area over whichthe detecting electrode directly faces the substrate, d_(h) is thedistance between the detecting electrode and the substrate, and e₀ isthe permittivity of vacuum.

From the equation (7), it can be seen that there are three methods toreduce the stray capacity C_(h): (1) a method of reducing the relativedielectric constant k_(h) of the insulator between the detectingelectrode and the substrate, (2) a method of reducing the area S_(h)over which the detecting electrode directly faces the substrate, and (3)a method of increasing the distance d_(h) between the detectingelectrode and the substrate. In the embodiment, the stray capacity C_(h)between the detecting electrode and the substrate is decreased by amethod in which a region with a dielectric constant less than that ofthe substrate is provided in a portion of the substrate such that themethod (2) of reducing the area over which the detecting electrodedirectly faces the substrate, or the method (3) of increasing thedistance between the detecting electrode and the substrate can beachieved.

Further, while the resistivity r of the substrate is determined by amaterial of the substrate, the resistivity of the entire substratecannot be directly increased in the embodiment because the embodimenthas the major premise that the substrate is formed of a material with arelatively small resistivity, such as a semiconductor with arelatively-high carrier concentration. However, when the structure ofthe substrate is subjected to appropriate means, it is possible toincrease a resistance R which is undergone by an AC signal leaking fromthe detecting electrode into the substrate through the stray capacityC_(h).

The resistance R experienced by the AC signal leaking into the substrateis given byR=r·L _(s) /S _(s)  (8)where L_(s) is the length of a path P of an electric signal from aportion of the substrate directly beneath a detecting electrode to aportion of the substrate directly beneath another detecting electrode,and S_(s) is the area of a cross section of the path P. It can beunderstood from the equation (8) that the following methods can be takenin order to increase the resistance R which is experienced by the ACsignal leaking from the detecting electrode into the substrate throughthe stray capacity. Those methods are (1) a method of replacing aportion of the path P by an insulator portion to locally increase theresistivity of the path P, (2) a method of increasing the length L_(s)of the path P, and (3) a method of reducing the area S_(s) of the crosssection of the path P. The embodiment selectively uses those methods.

A more specific structure of the first embodiment will now be describedwith reference to FIGS. 1 and 2. FIG. 2 illustrates a detailed structureof the substrate 100 in the electric potential measuring apparatus. Theinsulator layer or film 102 is formed between the substrate 100 and thedetecting electrode 103. Directly under the insulator layer 102, thereis provided a region 101 filled with an organic compound down to apredetermined depth, or a region whose dielectric constant is lowered.The organic compound is not exposed at a bottom surface of the substrate100. Electric connection from the detecting electrode 103 is carried outas described above. The output of the signal amplifier 107 issynchronously detected by the signal detecting circuit (CKT) using adriving frequency of the chopper 104. In connection with the signaldetecting circuit, an integrated circuit can be constructed on thesubstrate 100 when a semiconductor substrate, on which an integratedcircuit can be formed, is used as a main material of the substrate 100.Such an integral configuration on the substrate can achieve a smallerconstruction.

The dielectric constant e₂ (more accurately, an average value of thedielectric constant per unit volume) of the organic compound in theregion 101 is smaller than the dielectric constant e₁ of thesemiconductor substrate 100. Accordingly, the stray capacity created bythe detecting electrode 103 disposed on the region 101 is made smallerthan that in a case where the region 101 is a semiconductor substrateitself. As a result, a reduction in the impedance between the detectingelectrode 101 and the ground can be oppressed.

A fabrication method of the above structure will be described. A siliconis used as the semiconductor substrate 100. A hole is formed in aportion of the Si substrate corresponding to a portion directly beneaththe detecting electrode 103 to be formed thereafter, by using wetetching or dry etching. The hole is then filled with a polyimide of anorganic material, and the polyimide is hardened. The region 101 with areduced dielectric constant is thus formed. Further, insulator layer 102and detecting electrode 103 are formed on a surface of the region 101.The construction as illustrated in FIGS. 1 and 2 is thus fabricated.

The dielectric constant of the polyimide is about 3.2, while thedielectric constant of the silicon is about 11.9. Accordingly it ispossible to reduce the stray capacitor created by the detectingelectrode 103 on the region 101. Alternatively, SiO₂ can be formed inthe region 101 by oxidizing a portion of the silicon substrate.

The operation of the above construction is carried out in the followingmanner. Electric lines of force emitted from the measurement object 105are modulated by the vibration of the chopper functioning as thecapacity modulating unit 104. The detecting electrode 103 detects themodulated electric lines of force. The amount of charges induced in thedetecting electrode 103 is modulated by the electric lines of force, anda small AC current is generated from the detecting electrode 103. The ACcurrent is converted into a voltage by a large resistor connected to theground. A signal of the voltage amplified by the signal amplifier 107 isused for measurement of the electric potential of the measurement object105.

A second embodiment of the present invention will now be described withreference to FIG. 3. In the second embodiment, the region 101 directlyunder the detecting electrode 103 and the insulator layer 102 in thesemiconductor substrate 100 reaches a bottom surface of the substrate100. The region 101 is composed of a porous structure. In this case, avolumetric average value e₂A of the dielectric constant of the region101 is given bye ₂ A=e ₁ ·P  (9)where e₁ is the dielectric constant of the semiconductor substrate 100,and P is the packed ratio of the region 101. The packed ratio P isdefined by a ratio of a volume of a material portion, where the materialexists in the porous structure, relative to the total volume of theregion 101 including the material portion and a vacant portion.

Here, P is less than 1 (one), and therefore e₂A is smaller than e₁.Accordingly, the value of the stray capacity associated with thedetecting electrode 103 can be advantageously reduced.

A fabrication method of the above structure will be described. A siliconis prepared as the semiconductor substrate. A first portion of thesemiconductor substrate other than a second portion corresponding to theregion 101 is covered such that the first portion will not be influencedby anodic oxidation. The second portion of the semiconductor substrateis then subjected to the anodic oxidation to form the region 101 of theporous structure. After the anodic oxidation, the insulator layer 102and the detecting electrode 103 are formed on the region 101 of theporous structure.

Thus, the fabrication of an electric potential measuring apparatus isachieved. The construction as illustrated in FIG. 3 can be obtained bythe anodic oxidation performed to a desired portion of the substrate.Anodic oxidation can be carried out from upper and lower surfaces of thesubstrate to effectively fabricate the apparatus. The operation of thesecond embodiment is substantially the same as that of the firstembodiment.

A third embodiment of the present invention will now be described withreference to FIGS. 4A to 4C. The distance between a measurement objectand a detecting electrode is modulated in the third embodiment, whilethe number of electric lines of force reaching the detecting electrodeis modulated (i.e., S in the above equation (5) is modulated) by thechopper in the first and second embodiments. As can be understood fromthe equation (5), the coupling capacity can also be changed bymodulating the distance x.

FIG. 4C schematically shows the entire structure of an electricpotential measuring apparatus of the third embodiment. In FIG. 4C, theelectric potential measuring apparatus is disposed facing a measurementobject 105. Reference numeral 401 designates a case for encasing theelectric potential measuring apparatus. The case 401 covers an upperportion of a swinging member 100 except a portion of detectingelectrodes 103. The case 401 is formed of an electrically-conductivematerial, and connected to the ground. A supporting substrate 402 forrotatably supporting the swinging member 100 is fixed to the case 401 byan appropriate installing jig 403. The presence of the thus-arrangedcase 401 makes it possible that only electric lines of force from aportion of the measurement object 105 approximately directly facing theswinging member 100 reach the detecting electrodes 103. Thus, noisecomponents can be oppressed, and highly precise measurement of anelectric potential of the measurement object 105 can be ensured.

Further, an opening is formed in a central portion of the supportingsubstrate 402, and the planar swinging member 402 is rotatably supportedin this opening. The swinging member is supported in aswingingly-rotatable manner about a center axis C by a couple of torsionbars (front side and rear side) extending perpendicularly to the sheetof FIG. 4C. The swinging member 100 and the supporting substrate 402 areformed of a semiconductor material.

On an upper surface of the swinging member 100, two planar detectingelectrodes 103 with the same shape are disposed symmetrically withrespect to the center axis C. Electric wires for the detectingelectrodes 103 are formed on the swinging member 100, the torsion barand the supporting substrate 402 (see FIG. 4A). A region 101 with areduced dielectric constant is formed under each detecting electrode103. The region 101 has the above-described construction.

When the swinging member 100 is swingingly rotated about the center axisC, the two detecting electrodes 103 on the swinging member 100periodically go close to or away from the measurement object 105 in amutually-opposite phase. Accordingly, it is possible to differentiallyamplify modulated current signals from the detecting electrodes 103.Each detecting electrode 103 is electrically connected to a pull-outelectrode (not shown) formed on the supporting substrate 402 via theelectric wire formed on the torsion bar. For example, the detectingelectrodes 103 are electrically connected to inverting and non-invertinginput contacts of a differential amplifier disposed externally of thesupporting substrate 402, respectively. Thus, modulated currents fromthe detecting electrodes 103 are differentially amplified, andsynchronously detected by the detecting circuit, similar to the firstembodiment.

Only one detecting electrode can be provided on the swinging member 100(i.e., one of the detecting electrodes 103 is removed in the structureof FIG. 4C). In this case, the modulated current from the detectingelectrode does not undergo the differential amplification, and isinstead detected as described in the first and second embodiments.

In the electric potential measuring apparatus of the third embodiment,the swinging member 100 can be periodically swung about the center axisC defined by the torsion bar, by appropriately selecting aswinging-member driving mechanism, shapes and materials of the swingingmember 100 and the torsion bar, and the like. The swinging-memberdriving mechanism includes a magnet provided on a bottom surface of theswinging member 100, and an external coil which generates magneticfields when an AC current is caused to flow therein. The construction ofthe swinging-member driving mechanism is not limited to thatelectromagnetic type. A driving mechanism of an electrostatic orpiezoelectric type can also be used.

FIG. 4A is a perspective view illustrating the above-discussedconstruction viewed from another angle of view. FIG. 4B is across-sectional view, taken along line 4B–4B′ of FIG. 4A. In theswinging member 100, an insulator portion 101 or the region 101 isformed at a portion under each of the detecting electrodes 103A and 103Bthrough the insulator layer 102, extending in a direction of thicknessof the swinging member 100. Due to such a construction, it is possibleto reduce an area, over which the detecting electrodes 103A and 103Bdirectly face a surface of the swinging member 100, to about zero (0).Therefore, as can be understood from the equation (7), the straycapacity between the detecting electrodes 103A and 103B and the swingingmember 100 can be largely reduced, as compared with a case where thedetecting electrode 103 is provided on the swinging member 100 onlythrough the insulator layer 102.

Further, an electric path between the detecting electrodes 103A and 103Bis almost intercepted in the third embodiment. Hence, the resistance Rin the equation (8) increases largely. As a result, the impedance in theequation (6) between the detecting electrodes 103A and 103B can belargely increased. It is thus possible to prevent or minimize thephenomenon of mixture of AC signals between the detecting electrodes103A and 103B. Therefore, relatively-accurate electric potential signalscan be obtained even if the swinging member 100 of a semiconductor witha relatively low resistivity, or of an electric conductor is used. Inthe third embodiment, the insulator layer 102 can be omitted iffabrication of such a structure is possible, since the insulator portion101 is formed under each of the detecting electrodes 103A and 103B.

In the embodiment of FIGS. 4A to 4C, the insulator portion 101 is formedhalfway in the direction of thickness of the swinging member 100.However, it is also possible to adopt a structure of FIG. 5A in which aninsulator portion 101 is formed throughout the thickness of a swingingmember 100. As the thickness of the insulator portion 101 in theswinging member 100 increases, the impedance between the detectingelectrodes 103A and 103A increases. Accordingly, the phenomenon ofmixture of AC signals between the detecting electrodes 103A and 103B canbe prevented or minimized to a greater extent, and an electric potentialmeasurement signal can be obtained with more accuracy. Also in thestructure of FIG. 5A, the insulator layer 102 can be omitted iffabrication of such a structure is possible, since the insulator portion101 is formed under each of the detecting electrodes 103A and 103B.

Further, it is possible to adopt the structures of FIGS. 5B and 5C inwhich an insulator portion 101 is formed halfway in the direction ofthickness of the swinging member 100 from a bottom surface of theswinging member 100 opposite to the side of the detecting electrodes103. Also in such structures, the impedance between the detectingelectrodes 103A and 103B can be increased. In those structures of FIGS.5B and 5C, an insulator portion 101 is interposed between the swingingsubstrate 100 and the magnet 104B to intercept an electric path throughthe magnet 104B which has an electric conductivity. Further, theinsulation can be increased more greatly by providing a space 501 underthe insulator portion 101 formed in the swinging member 100, asillustrated in FIG. 5C.

A fourth embodiment of the present invention will now be described withreference to FIGS. 6A and 6B. FIG. 6B is a cross-sectional view, takenalong line 6B–6B′ of FIG. 6A which is a plan view. In an electricpotential measuring apparatus of the fourth embodiment, an insulatorportion 101 is formed in a portion of a swinging member 100 betweendetecting electrodes 103A and 103B to increase the resistance of theswinging member 100. The insulator portion 101 is formed in a portion ofthe swinging member 100 between the detecting electrodes 103A and 103Bthroughout the direction of thickness of the swinging member 100, and apermanent magnet 104 a is provided on a bottom surface of the swingingmember 100 through an insulator layer 110. Since the permanent magnet104 a also has an electric conductivity, the insulator portion 101formed on the bottom surface is necessary to intercept an electric paththrough the permanent magnet 104 a.

In such a structure, the length of an electric path from the detectingelectrode 103A to the detecting electrode 103B becomes longer by thelength of a path through torsion bars 106 and 107. Therefore, it can beseen from the equation (8) that the resistance from the detectingelectrode 103A to the detecting electrode 103B can be made longer thanthat in a case where no insulator portion is provided. As a result, theimpedance between the detecting electrodes 103A and 103B can be largelyincreased. It is thus possible to prevent or minimize the phenomenon ofmixture of AC signals between the detecting electrodes 103A and 103B,and obtain an electric potential measurement signal with relatively-highaccuracy.

Where it is difficult to form the insulator portion 101 throughout thedirection of thickness of the swinging member 100 as illustrated in FIG.6B, it is possible to form an insulator portion 101 halfway in thedirection of thickness of the swinging member 100 from a surface of theswinging member 100, on which the detecting electrodes 103A and 103B aredisposed, as illustrated in FIG. 6C.

Further, it is possible to form insulator portions 101 halfway in thedirection of thickness of the swinging member 100 from a surface of theswinging member 100, on which the detecting electrodes 103A and 103B aredisposed, and from its surface opposite thereto, as illustrated in FIGS.6D and 6E, respectively. In those structures of FIGS. 6D and 6E, thewidth of a low-resistance region becomes narrow in a portion where theinsulator portions 101 are formed, so that the resistance in theequation (8) in a portion of the swinging member 100 between thedetecting electrodes 103A and 103B increases.

In the construction of FIG. 6C, the impedance between the detectingelectrodes 103A and 103B can be increased more as the insulator portion101 becomes deeper. In the construction of FIG. 6E, the insulatorportions 101 are formed in a zigzag manner. Accordingly, alow-resistance region thereat can be narrowed, and elongated. Thus, avalue of the resistance in the equation (8) in a portion of the swingingmember 100 between the detecting electrodes 103A and 103B can be furtherincreased.

A fifth embodiment of the present invention will now be described withreference to FIGS. 7A and 7B, and FIGS. 8A and 8B. FIGS. 7B and 8B arecross-sectional views, taken along line 7B–7B′ and line 8B–8B′ of FIG.7A and FIG. 8A, which are plan views, respectively. In an electricpotential measuring apparatus of the fifth embodiment, an insulatorportion 101 is formed surrounding each of detecting electrodes 103A and103B, viewed from above, and extends throughout the direction ofthickness of the swinging member 100. In such a structure, an electricpath in the swinging member 100 between the detecting electrodes 103Aand 103B can be insulated on its halfway, and the impedance between thedetecting electrodes 103A and 103B can be largely increased. Also inthose structures, the permanent magnet 104B is provided on a bottomsurface of the swinging member 100 through an insulator portion 101 forthe above-described reason.

Where it is difficult to form the insulator portion 101 throughout thedirection of thickness of the swinging member 100 as illustrated in FIG.7B or 8B, it is possible to form an insulator portion 101 halfway in thedirection of thickness of the swinging member 100, as illustrated inFIGS. 7C and 7D, and FIG. 8C. In those structures of FIGS. 7C and 7D,and FIG. 8C, an electric path in the swinging member 100 between thedetecting electrodes 103A and 103B can be narrowed, and elongated. Thus,a value of the resistance in the equation (8) in a portion of theswinging member 100 between the detecting electrodes 103A and 103B canbe increased.

Further, in the construction illustrated in FIG. 8C, there are provideda structure for reducing the stray capacity between the swinging member100 and the detecting electrodes 103A and 103B, and a structure forincreasing a value of the resistance of the electric path between thedetecting electrodes 103A and 103B through the swinging member 100, thetorsion bars 106 and 107, and the supporting member 402. Due tofunctions of those structures, the stray capacity and the like can bemore effectively reduced, and noise can be effectively lowered.

A sixth embodiment of the present invention will now be described withreference to FIGS. 9A to 9D. FIGS. 9B to 9D are cross-sectional views,taken along lines 9B–9B′, 9C–9C′, and 9D–9D′ of FIG. 9A which is a planview, respectively. In an electric potential measuring apparatus of thesixth embodiment, an insulator portion 101 is formed extending over allof a swinging member 100, torsion bars 106 and 107, and a supportingmember 402. In such a structure, it is possible to increase a value ofthe resistance of an electric path from a detecting electrode 103A to adetecting electrode 103B through the torsion bars 106 and 107, and thesupporting member 402. Thus, the impedance between the detectingelectrodes 103A and 103B can be increased.

In each of the swinging member 100, the torsion bars 106 and 107, andthe supporting member 402, the configuration of the insulator portioncan be selected from any one of configurations described in the third,fourth and fifth embodiments. In the construction of the sixthembodiment, the insulator portion 101 is formed extending throughout thedirection of thickness of the swinging member 100, the torsion bars 106and 107, and the supporting member 402, as illustrated in FIGS. 9B to9D. Therefore, a value of the resistance of the electric path fromunderneath the detecting electrode 103A to the detecting electrode 103Bcan be largely increased.

Description will now be given for an image forming apparatus of aseventh embodiment using an electric potential measuring apparatus ofthe present invention, with reference to FIG. 10. In FIG. 10, referencenumeral 2101 designates an electric potential measuring apparatus of thepresent invention. Reference numeral 2102 designates an electrostaticcharging device. Reference numeral 2103 designates a signal processingdevice. Reference numeral 2104 designates a high-voltage generatingdevice. Reference numeral 2105 designates a light exposing device.Reference numeral 2106 designates a toner supplying device. Referencenumeral 2107 designates a transferring material conveying roller.

Reference numeral 2108 designates a photosensitive drum. Referencenumeral 2109 designates a transferring material sandwiched between thetransferring material conveying roller 2107 and the photosensitive drum2108.

An electric potential distribution on the photosensitive drum 2108 canbe measured when an output of the potential measuring apparatus 2101 ismonitored in synchronism with the rotation of the photosensitive drum2108. Unevenness of an image can be reduced when the electrostaticcharging device 2102 is controlled based on the thus-measured electricpotential distribution.

As illustrated in FIG. 10, the electrostatic charging device 2102, theelectric potential measuring apparatus 2101, the light exposing device2105, and the toner supplying device 2106 are arranged around thephotosensitive drum 2108. The electrostatic charging device 2102electrifies a surface of the drum 2108, and the surface of the drum 2108is exposed to light using the exposing device 2105 to form a latentimage on the drum 2108. Toner is attached to the latent image by thetoner supplying device 2106 to obtain a toner image. The toner image isthen transferred to the transferring material 2109 sandwiched betweenthe transferring material conveying roller 2107 and the photosensitivedrum 2108, and the toner on the transferring material 2109 is fixed.Image formation is achieved by executing those process steps.

In the above-discussed structure, a charged condition of the drum 2108is measured by the electric potential measuring apparatus 2101 capableof outputting an accurate signal with reduced noise, its signal isprocessed by the signal processing device 2103, and the electrostaticcharging device 2102 is controlled by feeding the processed signal backto the high-voltage generating device 2104. Thus, a stable electricalcharging of the drum 2108 is achieved such that a stable image formationcan be obtained.

Except as otherwise disclosed herein, the various components shown inoutline or in block form in the figures are individually well-known andtheir internal construction and operation are not critical either to themaking or using of the present invention or to a description of thebeast mode of the invention.

While the present invention has been described with respect to what ispresently considered to be the embodiments and examples, it is to beunderstood that the invention is not limited to the disclosedembodiments and examples. The present invention is intended to covervarious modifications and equivalent arrangements included within thespirit and the scope of the appended claims.

This application claims priority from Japanese Patent Applications No.2004-204531, filed Jul. 12, 2004, and No. 2004-297017, filed Oct. 8,2004, the contents of which are hereby incorporated by reference.

1. An electric potential measuring apparatus operable to measure anelectric potential of a measurement object, comprising: a substratedisposed facing the measurement object, the substrate being composed ofa material having a first dielectric constant, the substrate including aregion formed therein having an average value of a dielectric constantless than the first dielectric constant; a detecting electrode providedon a surface of the region and facing the measurement object; and acapacity modulating unit configured to modulate a coupling capacitybetween the detecting electrode and the measurement object.
 2. Anelectric potential measuring apparatus according to claim 1, wherein thesubstrate is composed of a semiconductor material.
 3. An electricpotential measuring apparatus according to claim 2, wherein the regionin the substrate is composed of one of an oxide and a nitride of thesemiconductor material.
 4. An electric potential measuring apparatusaccording to claim 2, wherein the region in the substrate is composed ofa semiconductor material different from the semiconductor material ofthe substrate.
 5. An electric potential measuring apparatus according toclaim 1, wherein the region in the substrate is composed of aninsulator.
 6. An electric potential measuring apparatus according toclaim 1, wherein the region in the substrate is composed of an organiccompound.
 7. An electric potential measuring apparatus according toclaim 1, wherein the substrate is composed of a material, and whereinthe region in the substrate is composed of the material of the substrateand having a porous structure.
 8. An image forming apparatus comprising:the electric potential measuring apparatus according to claim 1; and animage forming unit, wherein the electric potential measuring apparatusincludes a face, on which the detecting electrode is disposed, isarranged facing a face of the measurement object of the image formingunit, and wherein the image forming unit controls image formation, basedon a signal detected by the electric potential measuring apparatus.